Titan’s Ice Volcanoes Might Produce Stuff of Life

August 14, 2008

For almost
thirty years, scientists have known that complex carbon compounds called
tholins exist on comets and in the atmospheres of the outer planets.
Theoretically, tholins might interact with water in a process called hydrolysis
to produce complex molecules similar to those found on the early Earth.

On the
Earth, complex organic molecules are believed to have been an early step in theemergence
of life; such compounds are called prebiotic.

Titan, the
sixth and largest moon of the planet Saturn, is thought to be made largely of
ice. Some of that ice may melt during meteor impacts or in underground
processes, producing “ice volcanoes” that emit a “lava”
containing ammonia mixed with water.

Could tholins
formed in Titan’s
atmosphere react with liquid water temporarily exposed by meteor impacts or
ice volcanoes to produce potentially prebiotic complex organic molecules — before
the water freezes? Until this year, no one knew.

Into the
lab

Now,
laboratory research by Catherine Neish, a graduate student working on her
doctorate in planetary science at the University of Arizona, shows in the
journal Astrobiology that, over a period of days, compounds similar to
tholins can be hydrolyzed (which means to react with water) at near-freezing
temperatures.

Liquid
water exposed on Titan
is believed to persist for hundreds to thousands of years — plenty of time for
such reactions to take place.

Tantalizingly,
it has been suggested that a similar process may have happened on the early
Earth.

In her lab,
Neish created organic compounds similar to tholins
by subjecting a mixture of 5 percent methane and 95 percent nitrogen to
electrical discharge at a low temperature (-108 degrees F or -78 degrees C). She dissolved
samples of the resulting material in water, and then, at a range of
temperatures from freezing up to 104 degrees F (40 degrees C), measured the rate at which the
mixture hydrolyzed.

Neish found
that up to 10 percent of the organic compounds she began with reacted with
oxygen from the water to form complex
organic molecules.

While Neish’s
work was judged worthy of publication in a scientific journal, she has some
critics. James P. Ferris, a research professor at Rensselaer Polytechnic
Institute University, who has studied the chemistry of Titan’s atmosphere for
many years, calls her work “flawed” because she used an electric
discharge to generate tholins, while those in Titan’s atmosphere are probably
generated by ultraviolet (UV) light and charged-particle radiation.

Ferris has
conducted experiments on a mixture of gases similar to Titan’s atmosphere using
UV light and says, “The structures of the compounds made by [electric]
discharge differ from those formed by UV photolysis so the hydrolysis time
could be very different. Some of the photochemical products [when UV light is
used] are hydrocarbons that do not react with water.”

Neish responds
that electric, or plasma, discharge “was meant to mimic charged particle
interactions (which Ferris admits is a process at work on Titan).” She
agrees that “UV light radiation produces tholins that look more like Titan’s
haze,” but she points out that “some, if not most, of the products we
make also don’t react with water.”

She acknowledges
that her work is not an ideal representation of chemistry in Titan’s atmosphere:
“Tholins formed at low pressure seem to ‘look’ more like Titan’s haze than
those formed at higher pressures. You can make tholins at low pressures using
UV light; you cannot make tholins at low pressure using plasma discharge. And
to make the amount of tholins we needed for the experiment, we needed to use
the discharge technique. UV photolysis only produces small amounts.”

More to
come

Ferris, who
was not aware of Neish’s work until we contacted him, agrees that analyzing the
results of hydrolysis on samples produced by UV light would be “more
difficult because of the small samples formed.”

Another
issue is that Neish performed hydrolysis of her tholins in pure water, while
any water present on Titan is probably mixed with ammonia. She told us that she
recently completed another set of hydrolysis experiments using mixtures of
ammonia and water, and expects to publish those results shortly.

While Neish’s
work is not a perfect representation of chemistry on Saturn’s largest moon, it
nonetheless suggests that similar processes could produce organic compounds in
significant quantities during periods when liquid water is available.

On Titan,
this suggests that prebiotic molecules might exist in meltwater from impact
craters and ice
volcanoes. And similar processes might have occurred on the early Earth,
before our atmosphere contained significant quantities of free oxygen.